By Michael Martina and Ismail Shakil WASHINGTON (Reuters) – Family members of Palestinians killed in Israel’s military campaign in Gaza shared grief-ridden stories with thousands of protesters who gathered in downtown Washington on Saturday demanding an immediate ceasefire. In one of the largest pro-Palestinian demonstrations to date in the U.S. capital, the protesters repeated their call for U.S. President Joe Biden to stop sending arms to Israel and chanted “free Palestine” and “ceasefire now.” Some people chanted: “From the river to the sea, Palestine will be free” – a slogan that critics i…